Jupiter: Protection from Incoming Comets?

byPaul GilsteronAugust 24, 2007

Jupiter’s protective role for Earth has long been assumed, the theory being that the giant planet deflects asteroids and comets away from the inner Solar System. But studies on the subject are sparse, and focus on long period comets in extremely elliptical orbits. What about short period comets, and in particular the Jupiter Family of Comets (JFC)? These are comets thought to originate in the Kuiper Belt whose orbits are now controlled by Jupiter. Comet 81P/Wild 2, encountered by Stardust, was one of these, as was Comet Shoemaker Levy-9.

We all saw in 1994 what a planetary impact from a comet could do when Shoemaker Levy-9 struck Jupiter. And a new study suggests that Jupiter’s presence offers Earth no real protection from such objects. A research team at the UK’s Open University set up computer models that examined the Jupiter Family, defined for these purposes as comets whose semi-major axes are smaller than Jupiter’s and whose orbital period is less than twenty years. Some studies indicate that the so-called ‘Centaurs,’ icy objects which cross the orbits of the giant planets, are in transition between the Kuiper Belt and the Jupiter Family of Comets.

Image: Heavy bombardment. The role of Jupiter in slowing its pace is now being reassessed. Credit and copyright: Julian Baum.

Assuming the Centaurs as the parent population for the JFCs, the team set up a computer simulation that could track 100,000 Centaurs around the Solar System over a period of 10 million years. The model’s parameters were first adjusted to include a Jupiter at current mass, and the model was then run with planets at three-quarters, one-half, and one-quarter the mass of Jupiter. A final simulation was run with no planet in Jupiter’s position at all.

The result: The cometary impact rate with Jupiter in place is comparable to the rate when no planet exists in that position at all. What does change the rate of impacts is to assume a Jovian mass between these two extremes, in which case the rate goes up. Jonathan Horner (Open University), who presented these results at the European Planetary Science Congress in Potsdam today, has this to say:

“We’ve found that if a planet about the mass of Saturn or a bit larger occupied Jupiter’s place, then the number of impacts on Earth would increase. However if nothing was there at all, there wouldn’t be any difference from our current impact rate. Rather than it being a clear cut case that Jupiter acts as a shield, it seems that Jupiter almost gives with one hand and takes away with the other!”

The simulations indicate that without Jupiter, these short-period comets will not be diverted onto Earth-crossing orbits in the first place, whereas a Saturn-mass planet (about a third the mass of Jupiter) would have the gravitational pull to draw such comets into Earth-crossing orbits (as Jupiter does), but would not be massive enough to later eject them from the Solar System as readily as Jupiter. Thus we wind up with more objects on Earth-crossing orbits — and more impacts — at any given time.

Does the ‘Jupiter as shield’ hypothesis still hold up for long-period comets? And what role does the giant planet play in the risk of asteroid impact? The team is now at work on the asteroid question, and plans to address the long-period comets after that. But the notion that a solar system needs a gas giant analogous to Jupiter to protect its inner planets may need serious re-examination, even as we continue to study the danger posed by Earth-crossing objects of all kinds.

We seek to characterize giant-planet systems by their gravitational scattering properties. We do this to a given system by integrating it numerically along with a large number of hypothetical small bodies that are initially in eccentric habitable zone (HZ)-crossing orbits. Our analysis produces a single number, the escape rate, which represents the rate at which the small-body flux is perturbed away by the giant planets into orbits that no longer pose a threat to terrestrial planets inside the HZ. Obtaining the escape rate this way is similar to computing the largest Liapunov exponent as the exponential rate of divergence of two nearby orbits. For a terrestrial planet inside the HZ, the escape rate value quantifies the “protective” effect that the studied giant-planet system offers. Therefore, escape rates could provide information on whether certain giant-planet configurations produce a more desirable environment for life than the others. We present some computed escape rates on selected planetary systems, focusing on effects of varying the masses and semi-major axes of the giant planets. In the case of our Solar System we find rather surprisingly that Jupiter, in its current orbit, may provide a minimal amount of protection to the Earth.

Again follows on from Wetherills 1994 work on long-period comets.
(wishing I was in Potsdam at the minute).

If the results of this study are confirmed, it will be one more blow against the Rare Earth hypothesis. I personally think that, like the Ptolemaic universe, the Rare Earth hypothesis uses an excessive amount of special pleading (as you can see from my review of the eponymous Ward and Brownlee book at Rare Earth review

I begining to have doubts about the Rare Earth Hypothesis as well. I recently ran across some info on the internet suggesting that Mars had plate tectonics early in its history and that these went away when the interior cooled off. As far as anyone has been able to determine, Mars never had a large moon. Thus, plate tectonics may not necessitate the need for a large moon.

The large moon/plate tectonics was the part of the Rare Earth Hypothesis I thought most significant and unique to the theory.

Nonetheless, the REH was a respectable theory when it was first rolled out in 1999-2000, given the information we had at that time.

It appears that Mars did, indeed, have plate tectonics 4 billion years ago. Mars Global Surveyer has found the same kind of magnetic bands on Mars that were used to confirm the theory on Earth back in 1965. Have a look:

This then brings up how the population of Centaurs is generated: presumably they originate from the Kuiper belt, which suggests an investigation of how Neptune affects the outer solar system is also relevant to the generation of Earth-crossing objects.

Way premature to discount any Rare Earth hypothethis or to confirm it. We will have some real data regarding rarity in just a few more years once Kepler flies, now slipped to 2009. If Kepler flies, by 2012 it will either confirm or deny the common aspect of Earth sized planets in habitable zone orbits.

I thought that the current thoughts were that for plate tectonics to operate on a planet, you need a wet lithosphere as a dry lithosphere does not subduct as easily. From that, it would follow that as Mars and Venus lost their water, plate tectonics would cease, as it will on Earth when we lose our water.

However, the REH guys made a big deal out of the idea that a large moon, made by the giant impact, was necessary in order to initiate and sustain plate tectonics. If this is the case, then Earth-like planets are quite rare as this is probably a very rare event. This is the part of REH I thought most crucial in determining the occurance of Earth-like planets. However, if plate tectonics can occur on a world without a large moon, like the magnetic bands suggests of Mars, then Earth-like worlds are much more likely to occur than in the previous case, because plate tectonics is essential for a sustainable biosphere (this is the part of the REH that I definitely agree with).

The large moon is still necessary to stabilize the axial tilt of an “Earth”. However, this is not a show stopper to me because even if the planet’s axial tilt ranges from zero to 90deg, it can still be a habitable planet, even if it has no life beyond blue-green algae.

Just to add another stick on the funeral pyre of the REH. The July 2007 “Spaceflight” had a revealling interview with Robin Canup on Earth-Moon formation. Dr. Canup does high resolution modelling of late-stage accretion of the planets. She’s of the opinion that all the inner planets formed largish moons through collisions in the Solar System’s early days. All but Earth’s became unstable and either crashed back or floated away. Thus there’s nothing freakish about the Moon – end-stage accretion produces such things as a matter of course. We got lucky because ours survived, but not that lucky.

Kurt9: regarding tidal stabilisation of the planet, consider the case of a planet around a star less luminous than our Sun. Such a planet would be located closer to its star than Earth is, and thus experience stronger solar tides. From what I’ve seen, the discussions of solar tides in the context of habitable planets seems to only consider the case where the solar tides are so strong that they lock the planet into synchronous rotation, but for intermediate cases, solar tides can be comparable to the lunar tides Earth experiences. This would presumably be able to stabilise the planet’s rotation axis.

Adam: that’s an interesting view: I suppose that given the incidence of binaries in the Kuiper Belt it shouldn’t be all that surprising. Last I’d heard, it was thought that only a relatively small subset of giant impacts resulted in moon formation, but that most terrestrial planets undergo such collisions as part of late-stage accretion, but if moon formation is more common, it’s an interesting result, which could make hypotheses like this one about the lack of a Venusian moon more plausible.

Thats a valid point. However, would not such strong solar tides slow the planet’s rotation so that the “day” would be, say, 100-200 hours long? The day/night temperature swing ought to be impressive. On the other hand, if the PIQ (planet in question) is mostly ocean, that ocean ought to dampen the temerature swings suffficently to make it somewhat habitable. However, you would still have temperature swing (think of North Dakota on steroids).

About Canup’s modeling with the moons, I am usually skeptical of computer models, as apposed to actual data. However, we have 3 planets in or near the goldilocks zone (Venus, Earth, and Mars) giving us three data points. Two of those have or had plate tectonics in the past (Earth and Mars). So, 2 out of 3 data points with plate tectonics ain’t bad. So, even if Canup is wrong, the REH is likely irrelevant here.

Again, a lot can be learned by studying our own solar system, since we can actually go to these places using existing technology.

Kurt9: I don’t see why a solar tide of that magnitude would cause that kind of problem: after all, the lunar tides have been going for 4.5 billion years (and in fact were stronger in the past, due to the moon moving gradually outwards), yet we don’t have a 28-day rotation period. Then again, for the case of Earth, we do have the competing influence of solar tides to consider I suppose.

It depends how close you have the planet to the star (I’m assuming you are talking about a lower K type star). I have read that the solar tide on our Earth is about half that of the lunar tide. So, maybe that planet would not be so close to its primary that it can still have a normal day (say, less than 50-60 hours) and still have a tide equal to our lunar tide. This may be likely since the habitable zone around a K-star will be smaller than ours. I think computer modeling could easily determine this.

Basically tidal force is proportional to the mass of the perturbing body and inversely proportional to the cube of the separation, so the relative strength of lunar tides and solar tides can be calculated by:

(m_moon/m_sun) / (384400 km/1 AU)^3 = 2.2

To take an exoplanetary example, for 55 Cancri (G8V), the luminosity is about 0.61 times solar, which puts the habitable zone at approximately 0.78 AU (which is in the rather wide gap between planets “c” and “d”). The mass of the star is 0.95 times solar, which makes the tidal force on a habitable planet about twice that of the solar tide on Earth, or about 90% of the lunar tide on Earth.

So maybe late-G or early-K stars are the best bet for rotation-stabilised planets which don’t have extremely long day-night cycles. So much for solar analogues?

Andy: Certainly true. I think even if the planet’s axil tilt is not stabilized and there is very little solar tide or a long day/night cycle, that there could still be sophisticated life in the oceans with very little or simply land life.

I do think that when we get out there, we will find lots of “Earth-like” worlds that have complex ocean life, but no complex land life.

There’s been a lot of GCM work on synchronous rotating planets in tide-lock orbits. Oceans do moderate the temperature swings, but even a fairly thin atmosphere can prevent atmosphere freeze-out on the darkside. There’s been a reassessment of habitable planets around red-dwarf stars for that very reason. We should’ve realised once Venus’s ultra-slow diurnal cycle and near zero surface temperature variation was discovered, that atmospheres can transport a LOT of heat. Atmospheres self-organise into a really huge heat-pump from one side of the planet to the other.

So what would have to the geological cycling of materials in that case? I would suggest that geothermal cycling would continue, but be focussed – like a giant mantle “hot-spot” on two opposing sides of the planet. And tides from other planets in the system, via eccentricity pumping, would help keep things in motion. A planet has an internal heat-source that has to get out, thanks to thermodynamics, and any differential will provide a direction to that heat-flow. Perhaps a literal “ring of fire” will form all the way around the Terminator because that’s squeezed in closer to the core than the solar/anti-solar points?

Your description of plate tectonics on a rotationally-locked planet around an M-star is interesting. Again, there are analogs in our own solar system for studying this. That is, the Galilean moons of Jupiter. I think one or more of these has geological processes similiar to, but not the same as plate tectonics.

Andy and Adam,

I just looked at the Rare Earth Hypothesis website again. I actually think that their equation is reasonable, compared to the drake equation. Its just that the numbers (probabilities) assigned to each factor look to be a lot higher than Brownlee and Ward presume.

Based on stuff I have been reading on this site and others, I think its likely that there are a lot more “Earth-like” worlds out there than Brownlee and Ward have assumed. However, I think their assumptions were reasonable at the time they made them (around 1999, I think).

In any case, Brownlee and Ward make no assumptions of the evolutionary development of intelligence, once complex life does form. They leave this to the evolutionary biologists, which is an entirely different discussion.

SRT doesn’t have a gravity field. If there is no gravity
field , the space will be flat ( Pseudo- Euclid’s space),
but usually this space is called “Minkowski space “
(negative 4-D united space/time continuum).
======.
Is the “ Minkowski space “abstract continuum, as everybody says?
I think this space is a real one.
I think this space is Vacuum.
Why?
1. “ Minkowski space “has no gravity field, but negative parameter.
2. Only Vacuum space has negative parameter : T= – 273.
3. And this negative parameter is united with space/ time ,
which are joined together absolutely .
4. And the second SRT postulate tells about moving
light quanta in Vacuum.
5. It is impossible SRT to be the right theory
and space around SRT to be an abstract theory.
6. If in our brain abstract and real ideas are mixed together
then the interpretation of physics must be paradoxical.
============
P.S.
a) SRT is a right theory .
The bombs of Nagasaki and Hiroshima proved it.
But ” Minkowski space ” is an abstract theory.
There isn’t any proof of it existence.
b) Our planet Earth is home for us.
We live and act in this planet.
And ” Minkowski space ” is home for SRT.
All SRT particles live and act in this
” 4-D negative continuum – Minkowski space ” .
But nobody knows what ” Minkowski space ” is.
c) These two ideas are mixed together and therefore
the interpretation of physics is paradoxical.
========= ===========
SRT has only one space – “Minkowski space “.
But in 1915 Einstein put a “ MASS “ in the
“Minkowski space “ and it curved.
In 1921 A. Freedman put “ TIME “ in the
“Minkowski space “ and it also curved.
And Einstein had to agree with Freedman’s idea.
What is the reason of “Minkowski space “ change?
==========
If mathematician makes a small mistake in the
beginning of his calculations then after some
operations it grows into a big one.
And if in the beginning of sciences birth (Newton )
the abstract ideas were put into its fundament ,
then now we are surprised with its paradoxes………
……and we can create new and new theories for 1000 years
but the result will be the same – paradoxical.
===========================..

It began in 1905 when Einstein created SRT,
(theory of photon/electron’s behaviour).
Minkowski, tried to understand SRT using 4D space.
Poor young Einstein, reading Minkowski interpretation,
said, that now he couldn’t understand his own theory.

“ Einstein, you are right, it is difficult to understand SRT
using 4D space. But it is possible using my 5D space”
– said Kaluza in 1921.
This theory was tested and found insufficient.
“Well”, said another mathematicians, – “maybe 6D, 7D,
8D, 9D spaces will explain it”. And they had done it.
But the doubts still remain.
“OK”, they say, “we have only one way to solve this problem.
We must create more complex D spaces”.
And they do it, they use all their power, all their super intellects
to solve this problem.
Glory to these mathematicians !!!!
But……….
But there is one problem.
To create new D space, mathematicians must add a new parameter.
It is impossible to create new D space without a new parameter.
And the mathematicians take this parameter arbitrarily
(it fixed according to his opinion, not by objective rules).

The physicist, R. Lipin explained this situation in such way:
“Give me three parameters and I can fit an elephant.
With four I can make him wiggle his trunk…”
To this Lipin’s opinion it is possible to add:
“with one more parameter the elephant will fly.”
The mathematicians sell and we buy these theories.
Where are our brains?

Please remember, many D spaces were born as a wish
to understand SRT (theory of photon/electron’s behaviour).
But if someone wants to understand, for example, a bird
(photon/electron)itself and for this he studies only
its surroundings, will he be successful?

If I were a king, I would publish a law:
every mathematician who takes part in the creation
of 4D space and higher is to be awarded a medal
“To the winner over common sense”.
Why?
Because they have won us over using the
absurd ideas of Minkowski and Kaluza.
==============..http://www.socratus.com

Abstract: One of the mainstays of the controversial “rare Earth” hypothesis is the “Goldilocks problem” regarding various parameters describing a habitable planet, partially involving the role of mass extinctions and other catastrophic processes in biological evolution. Usually, this is construed as support for the uniqueness of the Earth’s biosphere and intelligent human life.

Here I argue that this is a misconstrual and that, on the contrary, observation-selection effects, when applied to catastrophic processes, make it very difficult for us to discern whether the terrestrial biosphere and evolutionary processes which created it are exceptional in the Milky Way or not. In particular, an anthropic overconfidence bias related to the temporal asymmetry of evolutionary processes appears when we try to straightforwardly estimate catastrophic risks from the past records on Earth. This agnosticism, in turn, supports the validity and significance of practical astrobiological and SETI research.

Abstract: We present photometry on 23 Jupiter Family Comets (JFCs) observed at large heliocentric distance, primarily using the 2.5m Isaac Newton Telescope (INT). Snap-shot images were taken of 17 comets, of which 5 were not detected, 3 were active and 9 were unresolved and apparently inactive. These include 103P/Hartley 2, the target of the NASA Deep Impact extended mission, EPOXI. For 6 comets we obtained time-series photometry and use this to constrain the shape and rotation period of these nuclei. The data are not of sufficient quantity or quality to measure precise rotation periods, but the time-series do allow us to measure accurate effective radii and surface colours. Of the comets observed over an extended period, 40P/Vaisala 1, 47P/Ashbrook-Jackson and P/2004 H2 (Larsen) showed faint activity which limited the study of the nucleus. Light-curves for 94P/Russell 4 and 121P/Shoemaker-Holt 2 reveal rotation periods of around 33 and 10 hours respectively, although in both cases these are not unique solutions. 94P was observed to have a large range in magnitudes implying that it is one of the most elongated nuclei known, with an axial ratio a/b \ge 3. 36P/Whipple was observed at 5 different epochs, with the INT and ESO’s 3.6m NTT, primarily in an attempt to confirm the preliminary short rotation period apparent in the first data set. The combined data set shows that the rotation period is actually longer than 24 hours. A measurement of the phase function of 36P’s nucleus gives a relatively steep \beta = 0.060 \pm 0.019. Finally, we discuss the distribution of surface colours observed in JFC nuclei, and show that it is possible to trace the evolution of colours from the Kuiper Belt Object (KBO) population to the JFC population by applying a ‘de-reddening’ function to the KBO colour distribution.

Abstract: The action sphere method program is written. The initial conditions set at pericenter of planetocentric orbits. When action sphere radius is reached, the heliocentric orbit is calculated and data redirected to numeric integration program. The method is useful for capture and collision problem investigation. The very preliminary numeric results were obtained and discussed. A manifold in orbital elements space, leads to temporary capture about 50 year (4 Jupiter revolutions), was found.

Abstract: The short period Jupiter family comets (JFCs) are thought to originate in the Kuiper Belt; specifically a dynamical subclass of the Kuiper Belt known as the `scattered disk’ is argued to be the dominant source of JFCs. However, the best estimates from observational surveys indicate that this source falls short by about three orders of magnitude the estimates obtained from theoretical models of the dynamical evolution of Kuiper belt objects into JFCs.

We re-examine the scattered disk as a source of the JFCs and make a rigorous estimate of the discrepancy. We find that the uncertainties in the dynamical models cannot account for the discrepancy. A change in the size distribution function of the scattered disk at faint magnitudes (small sizes) beyond the current observational limit offers a possible but problematic resolution to the discrepancy.

We discuss several other possibilities: that the present population of JFCs is a large fluctuation above their long term average, that larger scattered disk objects tidally break-up into multiple fragments during close planetary encounters as their orbits evolve from the trans-Neptune zone to near Jupiter, or that there are alternative source populations. Well-characterized observational investigations of the Centaurs, objects that are transitioning between the trans-Neptune Kuiper belt region and the inner solar system, can test the predictions of the non-steady state and the tidal break-up hypotheses.

Abstract: An interesting feature of the giant planets of our solar system is the existence of regions around these objects where no irregular satellites are observed. Surveys have shown that, around Jupiter, such a region extends from the outermost regular satellite Callisto, to the vicinity of Themisto, the innermost irregular satellite.

To understand the reason for the existence of such a satellite-void region, we have studied the dynamical evolution of Jovian irregulars by numerically integrating the orbits of several hundred test particles, distributed in a region between 30 and 80 Jupiter-radii, for different values of their semimajor axes, orbital eccentricities, and inclinations.

As expected, our simulations indicate that objects in or close to the influence zones of the Galilean satellites become unstable because of interactions with Ganymede and Callisto. However, these perturbations cannot account for the lack of irregular satellites in the entire region between Callisto and Themisto.

It is suggested that at distances between 60 and 80 Jupiter-radii, Ganymede and Callisto may have long-term perturbative effects, which may require the integrations to be extended to times much longer than 10 Myr. The interactions of irregular satellites with protosatellites of Jupiter at the time of the formation of Jovian regulars may also be a destabilizing mechanism in this region.

We present the results of our numerical simulations and discuss their applicability to similar satellite void-regions around other giant planets.

Abstract: The asteroids are the major source of potential impactors on the Earth today. It has long been assumed that the giant planet Jupiter acts as a shield, significantly lowering the impact rate on the Earth from both cometary and asteroidal bodies. Such shielding, it is claimed, enabled the development and evolution of life in a collisional environment which is not overly hostile. The reduced frequency of impacts, and of related mass extinctions, would have allowed life the time to thrive, where it would otherwise have been suppressed. However, in the past, little work has been carried out to examine the validity of this idea.

In the first of several papers, we examine the degree to which the impact risk resulting from a population representative of the asteroids is enhanced or lessened by the presence of a giant planet, in an attempt to fully understand the impact regime under which life on Earth has developed.

Our results show that the situation is far less clear cut that has previously been assumed – for example, the presence of a giant planet can act to enhance significantly the impact rate of asteroids at the Earth.

Comments: 34 pages, 15 Figures, to appear in the International Journal of Astrobiology

The Composition of Dust in Jupiter-Family Comets as Inferred from Infrared Spectroscopy

Authors: Michael S. Kelley, Diane H. Wooden

(Submitted on 24 Nov 2008)

Abstract: We review the composition of Jupiter-family comet dust as inferred from infrared spectroscopy. We find that Jupiter-family comets have 10 micron silicate emission features with fluxes roughly 20-25% over the dust continuum (emission strength 1.20-1.25), similar to the weakest silicate features in Oort Cloud comets.

We discuss the grain properties that change the silicate emission feature strength (composition, size, and structure/shape), and emphasize that thermal emission from the comet nucleus can have significant influence on the derived silicate emission strength.

Recent evidence suggests that porosity is the dominant parameter, although more observations and models of silicates in Jupiter-family comets are needed to determine if a consistent set of grain parameters can explain their weak silicate emission features.

Models of 8 m telescope and Spitzer Space Telescope observations have shown that Jupiter-family comets have crystalline silicates with abundances similar to or less than those found in Oort Cloud comets, although the crystalline silicate mineralogy of comets 9P/Tempel and C/1995 O1 (Hale-Bopp) differ from each other in Mg and Fe content. The heterogeneity of comet nuclei can also be assessed with mid-infrared spectroscopy, and we review the evidence for heterogeneous dust properties in the nucleus of comet 9P/Tempel.

Models of dust formation, mixing in the solar nebula, and comet formation must be able to explain the observed range of Mg and Fe content and the heterogeneity of comet 9P/Tempel, although more work is needed in order to understand to what extent do comets 9P/Tempel and Hale-Bopp represent comets as a whole.

Abstract: It has long been assumed that the planet Jupiter acts as a giant shield, significantly lowering the impact rate of minor bodies upon the Earth, and thus enabling the development and evolution of life in a collisional environment which is not overly hostile.

However, in the past, little work has been carried out to examine the validity of this idea.

In the second of a series of papers, we examine the degree to which the impact risk resulting from objects on Centaur-like orbits is affected by the presence of a giant planet, in a continuing attempt to fully understand the impact regime under which life on Earth has developed.

The Centaurs, which occupy orbits beyond Jupiter, have their origins in the Edgeworth-Kuiper belt that extends beyond Neptune. The giant planets peturb the Centaurs, sending a significanr fraction into the inner Solar System where they become visible as short-period comets.

In this work we present results which show that the presence of a giant planet can act to significantly change the impact rate of short-period comets on the Earth, and that a giant planet often actually increases the impact flux greatly over that which would be expected were it not present. (Shortened version of abstract.)

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last twelve years, this site coordinated its efforts with the Tau Zero Foundation. It now serves as an independent forum for deep space news and ideas. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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